M2 macrophage infiltrates in the early stages of ANCA-associated pauci-immune necrotizing GN.

BACKGROUND AND OBJECTIVES This study examined kidney biopsies with focal segmental glomerular fibrinoid necrosis to identify early features of pauci-immune necrotizing GN and the primary effector cells mediating initial capillary injury. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS Seventeen consecutive kidney biopsies with focal pauci-immune necrotizing GN, obtained over a 6-year period (2007-2012), were studied. Neutrophils and CD68(+), CD163(+), CD3(+), CD56(+), and CD20(+) cells were scored in paraffin sections counterstained with periodic acid-Schiff. Electron microscopy was performed in 15 of 17 biopsies and additional examples of pauci-immune necrotizing GN (n=25). Biopsies with thin basement membrane nephropathy (n=5) served as immunohistologic normal controls. RESULTS Biopsies with pauci-immune necrotizing GN had a mean of 10 (range=3-25) normal-appearing glomeruli, a mean of 2 (range=1-5) glomeruli with segmental fibrinoid necrosis, and a mean of 2 (range=1-11) glomeruli with cellular crescents. CD68(+) and CD163(+) macrophages predominated at sites of fibrinoid necrosis in pauci-immune necrotizing GN, exceeding the quantity of neutrophils and T cells (mean scores [SD]=2.5 [0.7] and 2.2 [0.75] versus 0.6 [0.5] and 0.1 [0.3], respectively; P<0.001). B and natural killer cells were rare. Normal-appearing glomeruli in pauci-immune necrotizing GN had significantly more CD68(+) and CD163(+) macrophages than the controls (CD68(+), 0.9 [0.3] versus 0.4 [0.3]; CD163(+), 1 [0.4] versus 0.4 [0.3]; P<0.001). The quantity of other glomerular infiltrates did not differ from controls. The serum creatinine level at biopsy correlated with the glomerular CD68 and neutrophil scores (r=0.74 and r=0.71, respectively; P=0.001) but did not correlate with the extent of fibrinoid necrosis (r=0.36). Macrophages were localized at minute perforations and attenuations of the capillary basement membrane by electron microscopy. CONCLUSIONS Early pauci-immune necrotizing GN is characterized by a selective localization of CD163(+) M2 macrophages at sites of glomerular fibrinoid necrosis and in normal-appearing glomeruli. These observations indicate that alternatively activated macrophages are positioned as potential effectors of glomerular injury in the early stages of pauci-immune necrotizing GN and may be potential targets for therapeutic intervention.